Miniaturization and densification of a packaging method in a field of electronic devices for communication, consumer use, industry, and the like are significant in recent years, and excellent heat resistance, dimensional stability, and electrical properties are required for materials. For example, a printed wiring board conventionally employs a copper-clad laminate formed of a heat-curable resin such as a phenol resin or an epoxy resin as a material. Such a resin balances various properties but has a disadvantage of poor electrical properties, in particular, poor dielectric properties in a high frequency region. As a new material for solving the above-mentioned problem, polyphenylene ether has attracted attention recently and has been applied to a copper-clad laminate board and the like.
Polyphenylene ether refers to an engineering plastic having excellent mechanical properties and electrical properties and relatively high heat resistance. However, in the case where polyphenylene ether is to be used as material for a printed wiring board, the material is required to have very high solder heat resistance and intrinsic heat resistance of polyphenylene ether is not sufficient. That is, polyphenylene ether deforms in exposure to high temperatures of 200° C. or higher, to thereby cause significant reduction in mechanical strength or peeling of a copper foil formed as a circuit on a resin surface. Further, polyphenylene ether has high resistance to an acid, an alkali, and hot water but very low resistance to an aromatic hydrocarbon compound or a halogen-substituted hydrocarbon compound and dissolves in such a solvent.
As a method of improving heat resistance and chemical resistance of polyphenylene ether, there is proposed a method involving introducing a crosslinking functional group into a chain of polyphenylene ether, curing the whole, and using polyphenylene ether as cured polyphenylene ether. However, no satisfying solution has been obtained so far.
To be specific, in U.S. Pat. No. 3,281,393 and U.S. Pat. No. 3,422,062, cured polyphenylene ether is obtained by producing polyphenylene ether having an allyl group through copolymerization of 2-allyl-6-methylphenol and 2,6-dimethylphenol, and curing polyphenylene ether. However, polyphenylene ether having an allyl group has a melting temperature higher than a curing temperature, and thus it is impossible to perform heat forming such as vacuum lamination. As a method of improving such formability, use of a large amount of a plasticizer in combination has been attempted in U.S. Pat. No. 3,422,062, but the use of a plasticizer not only degrades excellent electrical properties (low dielectric constant and low dielectric dissipation factor) of polyphenylene ether and also leads to reduction in heat resistance and chemical resistance after curing.
Meanwhile, U.S. Pat. No. 4,634,742 discloses a method involving: obtaining curable polyphenylene ether by using a polymer of 2,6-dimethylphenol, and converting a methyl group into a vinyl group or introducing a vinyl group into a 3- or 5- position of a phenyl group; and heat-curing the resultant. In this case, the vinyl group is bonded directly to an aromatic ring of polyphenylene ether without a curved carbon chain or an ether bond. Thus, polyphenylene ether after curing has insufficient flexibility and becomes a very brittle material which is not suitable for practical use.
Examples of the prior documents related to the present invention include the following.
Patent Document 1: JP-A-06-179734
Patent Document 2: JP-A-2003-261743
Patent Document 3: JP-A-2003-292570
Patent Document 4: JP-A-2000-128908
Another method of using polyphenylene ether involves mixing a curable polymer or monomer into a polyphenylene ether resin. Polyphenylene ether is used in combination with the curable polymer or monomer, to thereby improve chemical resistance of polyphenylene ether and obtain a material having excellent dielectric properties of polyphenylene ether. Examples of the curable polymer or monomer include: an epoxy resin; 1,2-polybutadiene; a polyfunctional maleimide; a polyfunctional cyanate; a polyfunctional acryloyl compound; and triallyl isocyanurate.
JP-A-06-179734 discloses a curable composite material formed of: a reaction product of polyphenylene ether and an unsaturated carboxylic acid or the like (a); diallylphthalate, divinylbenzene, a polyfunctional acryloyl compound, a poly functional methacryloyl compound, a polyfunctional maleimide, a polyfunctional cyanate, a polyfunctional isocyanate, an unsaturated polyester (b), or the like; a thermoplastic resin (c); and a substrate (d). JP-A-06-179734 discloses a use of divinylbenzene or a prepolymer thereof as the component (b), but examples only disclose use of a reaction product of polyphenylene ether, and an unsaturated carboxylic acid or an unsaturated carboxylic anhydride as the component (a) and a use of divinylbenzene as the component (b). A curable composition produced through this method has low compatibility between the component (a) and the component (b). Thus, a cured product obtained from the curable composition has disadvantages of insufficient heat resistance, appearance, chemical resistance, and mechanical properties, and has problems in an industrial use such as a narrow range of working conditions and varying mechanical properties of a product.
JP-A-2003-261743 and JP-A-2003-292570 each disclose a reactive polyphenylene ether oligomer having a cyanate group or an epoxy group on each terminal, but includes no description of an oligomer having a vinyl group. In JP-A-2003-261743 and JP-A-2003-292570, no synergetic effect of a reactive polyphenylene ether oligomer having a vinyl group on each terminal and a soluble polyfunctional vinyl aromatic copolymer has been imagined for developing excellent formability and favorable compatibility and providing a high performance curable resin composition. Further, JP-A-2000-128908 discloses a method of obtaining a styrene-based polymer from a polyfunctional vinyl compound, a polyfunctional chain transfer agent, and a styrene-based monomer. However, JP-A-2000-128908 discloses a method involving adding a polyfunctional vinyl compound in a low concentration of 2,000 ppm or less, and the method is solely appropriate for application of a thermoplastic resin. Thus, no production of soluble polyfunctional vinyl aromatic copolymer and application of a heat-curable resin have been imagined from the technique disclosed in JP-A-2000-128908, and JP-A-2000-128908 includes no description of mixing with a heat-curable resin for use.
Thus, no production of a curable resin composition formed of a reactive polyphenylene ether oligomer having a vinyl group on each terminal and a soluble polyfunctional vinyl aromatic polymer, having favorable formability based on a high level of fluidity, and having favorable compatibility through control of a molecular weight, a molecular weight distribution, and a copolymer composition has been imagined as a material solving various problems of conventional techniques and used in high technology fields.